202 XIII INTEGRATION spaces «>fc(X; K) (with respect to the norm ||/||); in other words, the re- striction of // to <>f C(X) is a (complex) measure v. Furthermore, it follows from the inequality \u(f)\ g c - N^/), where /e Jfc(X), and from (13.3.2.1) that |v|(|/|) ^ c • N^/) = c - Xl/l). Hence we deduce from (13.15.1) and (13.15.3) that there exists a locally /Mntegrable function g0 such that \g0(x)\ ^ c almost everywhere with respect to /j., and such that |v| = g0 - JJL. If v = h - |v| (13.16.3) we have therefore v = g • fi, where g = g0h (13.14.5), and it is clear that g e &% '(//). This being so, the linear forms u and 6g agree on the subspace JTC(X), which is dense in &lc (13.11.6); since they are both continuous on JS?£, it follows that they are equal (3.15.2). Finally, the fact that Qgi = 602 implies g^ = g2 follows from (13.15.3). Q.E.D. It follows that the mapping g\-+9gisa linear isometry of the Banach space L£ onto the dual of the Banach space L£ (12.15). We remark also that it follows from (13.12.5) that, for each function /eJS?c(X, ft), the linear form g^-ifgdii on «£?£p(X, //) is continuous with respect to the topology defined by the seminorm N^ . But in general there exist linear forms on JS?<? (X, /A) which are continuous with respect to this topology but are not of the above type. PROBLEMS 1. (a) Let X be a locally compact space, /z a positive measure on X. Let/be a fi-measur- able mapping of X into R and p a real number such that .1 <^p <| -h oo. Show that r* Np(/) = sup \fg\ d^y where g runs through the set of functions belonging to «^*R(X) such that Nq(#) g 1, where g = p/(p — 1) (and q = -f oo if p — 1). State and prove an analogous result for a ft-measurable mapping of X into C. (b) Suppose that 1 < p< -f oo. For each function # e ^l(X, p) (resp. ^ e JSfi(X, JLI)), let^J denote the linear form f\~->^(Jg) on L|(X, /*) (resp. L£(X, ft)). Show that 5 »-»• 6g is an isometry of L&(X, /z) (resp. L^(X, jit)) onto the Banach space (5.7.3) dual to the Banach space L|(X, /z) (resp. Lfc(X, /x)). (Imitate the proof of (13.17.1), using (a),) 2. Let X be a compact space, p, a positive measure on X, and (/,) an orthonormal sequence in xyyi, ). Write K.(*. r) - EAWAW^ H.(J) = f |KW(5, *=i Jx (the "//th Lebesgue function" of the system (/„), cf. Section 11.6, Problem 2). For any g e ^(X, ft), let JB(^) denote the functionnce of complex measures on X, there